fix signature for nullability

and avoid warning

Author: StephaneDelcroix

snippets/csharp/System/EventArgs/Overview/programwithdata.cs

@@ -18,7 +18,7 @@ static void Main(string[] args)
             }
         }
 
-        static void c_ThresholdReached(object sender, ThresholdReachedEventArgs e)
+        static void c_ThresholdReached(object? sender, ThresholdReachedEventArgs e)
         {
             Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold,  e.TimeReached);
             Environment.Exit(0);